Remove deprecated mixin
Remove deprecated mixin

See themes/docsy/assets/vendor/bootstrap/site/content/docs/4.3/migration.md

> - Removed `hover`, `hover-focus`, `plain-hover-focus`, and `hover-focus-active` mixins. Use regular CSS syntax for these moving forward. [See #28267](https://github.com/twbs/bootstrap/pull/28267).
grpc/grpc.io-docsy,grpc/grpc.io-docsy,google/docsy,google/docsy